(1) Field of the Invention
This invention relates to a metal halide lamp, and more particularly to a metal halide lamp utilized for a projection display such as a liquid crystal projection display, and for a luminance mainly incorporated into a reflector and the like.
(2) Description of the Prior Art
Liquid crystal projection systems have been gaining popularity as a display means for magnifying and projecting characters or graphics on the screen.
This type of apparatus typically has such a construction that a light from a light source lamp is projected into a liquid crystal panel via a reflector and subsequently the light is projected onto a screen via a focusing optical system, which is a projecting optical system. Having such a construction, this type of apparatus can utilize only the light emitted from a limited region adjacent to the focal point of the reflector. It is therefore desirable that, in a lamp for the light source, the light emission by arc be concentrated in as small an area as possible. The reason is that the. efficiency in utilizing light increases as the light emitting area becomes smaller, thus achieving high illuminance at the screen. This tendency becomes more apparent as reduction of the physical sizes of reflectors and the like advances in the attempts to reduce the size, weight, and cost of the projection devices.
It is also desirable that the lamp used for the light source of projection systems have well-balanced light emission throughout the whole visible range of spectrum. In other words, if the lamp exhibits well-balanced light emission in each red, green, and blue region of spectrum, the projection system employing the lamp will be thereby able to reproduce, for example, an image of healthy human""s complexion as it should be. However, if the light emission in the red region is insufficient, the displayed image of the healthy human""s complexion turns out to be bluish, thus pale and unhealthy.
In consideration of the above, metal halide lamps, ultra high-pressure mercury lamps, or the like have been used for conventional liquid crystal projection displays and the like.
Metal halide lamps are a high-pressure discharge lamp characterized in that various types of metal halides are contained in high-pressure mercury vapor. One type of such a lamp is disclosed on pp. 18-24 of Characteristics and Theoretical Analysis of Metal Halide Lamps, T. Higashi, The Journal of the Illuminating Engineering Institute of Japan, Vol. 73, No. 9, 1989. The lamp includes the iodides of Sc (scandium) and Na (sodium) in the fill material, and exhibits a high luminous efficacy of 90 lm/W. (It is to be understood that the term xe2x80x98luminous efficacyxe2x80x99 herein means a luminous flux per unit input electric power to a lamp.) The reason for this is considered to be that a complex iodide possibly Na2ScI5), which has a higher vapor pressure than that of Sc and Na as a simple body, is formed from Sc and Na therein. (See pp. 209-214 of Complex Halide Vapors in Metal Halide Type HID Lamps, C. Hirayama et al., The Journal of the Illuminating Engineering Society, July 1977). The special distribution of this lamp is shown in FIG. 7. As seen from FIG. 7, a large number of bright-line spectrums are observed in the visible range, which indicates that the lamp has relatively high color rendering properties.
In the process of completing the invention, the present inventors experimentally produced a metal halide lamp having a construction described below and shown in FIG. 8. The lamp has an arc tube 101, composed of a light-transmissive quartz vessel having an approximately spherical shape, an inner diameter of 10.8 mm, and an inner capacity of 0.7 cc. Each of the opposite ends of the arc tube 101 is sealed at a seal portion 106. A pair of tungsten electrodes 102 is provided within the arc tube 101. Each of the tungsten electrodes 102 is connected to an external lead 104 via a molybdenum foil 103. A tungsten coil 105 is also connected to each of the tungsten electrodes 102 by welding. The gap between the terminal ends of the electrodes 102 (the distance between the electrodes) is set at 2.2 mm. A fill material 107 is enclosed in the arc tube 101. The fill material 107 comprises 0.6 mg of InI (indium iodide), 1 mg of TmI3 (thulium iodide), argon with 0.2 atm at a room temperature, and 49 mg of mercury.
The luminous efficacy of the lamp according to the above-described construction was about 80 lm/W when the lamp was disposed horizontally and operated at a rated input power. The luminous flux that reaches a 40-inch screen was measured under the condition where the light emitted from the lamp is projected with a taking angle of 7 degrees via an ellipsoidal reflector. The size of the luminous flux per unit input power was 4 lm/W. The size of the luminous flux per unit power measured according to the above-described manner is hereinafter referred to as xe2x80x98projection efficiencyxe2x80x99. It is to be noted here that conventional well known metal halide lamps have a longer distance between the electrodes (for example, approx. 3 mm), and therefore exhibit even lower projection efficiency than the above-described lamp. As to the spectral distribution, the lamp exhibited abundant light emission over the whole visible range, as shown in FIG. 9. In particular, the light emission in the red region of spectrum was more abundant than that of the previously-mentioned metal halide lamp comprising iodides of Sc and Na, which leads to more favorable color reproduction properties when the lamp is used for projecting image and the like.
An ultra high-pressure mercury lamp, for example, as the one described in Japanese Unexamined Patent Publication No. 2-148561, generally has such a construction that mercury is primarily included in the fill material and the vapor pressure of the mercury becomes very high during the operation. Halides of other metals are not included therein. An ultra high-pressure mercury lamp of this type exhibited a luminous efficacy of about 60 lm/W and a projection efficiency of 11 lm/W, when operated at a rated power. The spectral distribution of this lamp is shown in FIG. 10. Since this type of ultra high-pressure mercury lamp is operated with high vapor pressure, the light emission in the red region of spectrum, the wavelength range of around 600 to 650 nm, is a little improved over other types of mercury lamps which is operated with lower vapor pressure. Nonetheless, the amount of light emission in the red region of around 600 to 650 nm is still obviously smaller than that of the metal halide lamps mentioned above.
Now, the drawbacks of these prior art lamps will be further detailed below.
Although the above-described experimental metal halide lamp has relatively high luminous efficacy, it has a drawback in that the lamp cannot achieve high projection efficiency. This is due to the difficulty of making the light emitting area smaller. In consideration of this, as an index to indicate the size of the light emitting area, are diameters were measured for those lamps. From the results, it was confirmed that the experimental metal halide lamp containing In had a larger arc diameter of 1.1 mm than the ultra high-pressure mercury lamp, whose arc diameter was 0.7 mm. The metal halide lamp containing Na too has a drawback of larger arc diameter than the ultra high-pressure mercury lamp. Hence, these lamps cannot attain sufficient brightness at the screen in case where the lamps have a small reflector or a small to angle for the projection lens in the projecting optical system The reason for a large arc diameter in these lamps is that alkali metals such as Na and the like have low ionization potential as a simple body, for example, a ionization potential of Na being 5.14 eV, therefore easily ionize even in the low-temperature, peripheral area of the arc in the lamps. The alkali metals therefore generate free electrons, resulting in a wide electric current path, i.e., resulting in a large arc diameter. This is detailed on p.220 of Electric Discharge Lamps, John F. Waymouth, The MIT Press.
On the other hand, as mentioned above, the ultra high-pressure mercury lamp has a projection efficiency of 11 lm/W, and this is about three times the projection efficiency of the above-described metal halide lamp. However, although the light emission in the red region of spectrum is a little more improved than conventional mercury lamps, the ultra high-pressure mercury lamp cannot attain favorable well-balanced light emission over the whole visible range as can be achieved by metal halide lamps, since the luminophor thereof is limited to mercury.
In view of the above-mentioned problems, it is an object of the present invention to provide a metal halide lamp having a small arc diameter, high projection efficiency, and well-balanced light emission in terms of spectral distribution.
In accordance with the present invention, these and other objects are accomplished in a metal halide lamp having an arc tube in which a pair of electrodes is provided and a fill material comprising a rare gas and metallic elements including mercury is enclosed, the lamp being characterized in that each of the aforementioned metallic elements have a simple body ionization potential of 6 eV or higher, and that a distance between the terminal ends of the electrodes is set at such a distance that electric discharge therebetween is stably carried out, and that a distance between each of the terminal ends of the electrodes and the inner wall of the arc tube is set at not less than 1.5 times the distance between the terminal ends of the electrodes.
According to the above construction, thin arc is formed in the lamp by including in the fill material only the metallic elements with an ionization potential of 6 eV or higher. The lamp can thus attain high luminance and high projection efficiency, and high illuminance at a screen is thereby achieved In addition, unlike mercury lamps, the lamp according to the above construction can obtain a high level of color rendering properties with a favorable spectral distribution over the whole visible range of spectrum, since the luminophor is not limited to mercury as in mercury lamps.
In prior art metal halide lamps, Na or the like is added in the fill material in order to stabilize arc, However, it is considered that this is only necessary in the case where the distance between the electrodes is relatively long, e.g., approximately 10 mm. As the result of various experiments, the inventors have found that the formation of stable arc can be realized by restricting the distance between the electrodes at 2.5 mm or smaller, or preferably 2.0 mm or smaller, even if Na or the like is not added in the fill material, and that high luminance can be thereby achieved despite a low vapor pressure due to the absence of Na or the like.
Japanese Patent Publication No. 63-62066 discloses a lamp in which alkali metals are not included and the distance between terminal ends of the electrodes is made equal to the distance between the tube wall to the terminal ends of the electrodes. This technique is intended to realize stabilization of arc by the effect of the tube wall, and is effective for relatively low wattage lamps, for example the lamps with an input power of 50 to 70 W. However, for higher wattage lamps with a relatively short distance between the electrodes, this technique is not applicable since it leads to damage to the tube wall. By contrast, according to the present invention, the tube wall is kept away from the electrodes so that an input power to the lamp can be increased. In addition, according to the present invention, the distance between the electrodes is made short, and thus the invention can achieve the stabilization of arc and the increase in light emission.
It has been known in the art that the length of arc can be made short by making short the distance between the electrodes. However, it has not yet been made possible to extremely shorten the distance between the electrodes since such a short distance induces deterioration in operational life of the lamp.
On the other hand, the metal halide lamp according to the present invention requires smaller electric current than prior art metal halide lamps when operated at the same electric power More specifically, for example, assuming that the distance between the electrodes is 2 mm, in the case of the fill material including ScI3 and NaI, the voltage between the electrodes is about 40 V, and therefore the required electric current is 5 A in order to attain an input power of 200 W. By contrast, in the case of the fill material not including NaI the voltage between the electrodes is about 60 V, and therefore the required electric current becomes 3.3 A, which is obviously smaller than the above case, to attain the same input power of 200 W. Hence, the present invention makes it possible to set a short distance between the electrodes, which serves to generate stable arc, without causing deterioration in the operational life of the lamp.
It is preferable that the above-mentioned metallic elements with an ionization potential of 6 eV or higher have the following properties.
1) High vapor pressure
2) Strong light emission in the visible range and well-balanced light emission
3) High ionization potential as a simple body
For example, scandium can be employed for such a metallic element Scandium serves for a light emission around the wavelength of 630 nm, and therefore it is made possible by employing scandium to obtain a spectral distribution characteristic with abundant light emission in the red color region, the wavelength range of 600 to 650 nm. It is preferable that the scandium is in a halide form such as scandium iodide (ScI3 and scandium bromide (ScBr3) so that the enclosing of the scandium into the arc tube can be facilitated.
In addition, halides of rare-earth elements such as thulium iodide and the like may be enclosed in the arc tube so that the spectral distribution characteristic is further improved.
Furthermore, a light transmissive quartz tube may be employed for the arc tube. The light transmissive quartz tube has high transparency and small light dissipation compared to a ceramic tube for example, and therefore the advantage of small light emitting area achieved by thin arc becomes more apparent.